CARSON, Calif. - When Robbie Keane firmly tapped the Los Angeles Galaxy's third goal into the net, their roaring fans could be excused for thinking ahead to another MLS Cup celebration.

Even after opening the Western Conference finals in dominant style, Keane and the surging Galaxy say they won't plan for another title shot before they finish this job.

Keane scored twice, Mike Magee added his latest clutch playoff goal and Los Angeles beat the Seattle Sounders 3-0 on Sunday night to take a commanding lead in the first leg of the conference finals.

Keane had an assist and Sean Franklin set up two goals by the defending champions, who will head to Seattle next Sunday in prime position to earn their third MLS Cup appearance in four years.

"We had a wonderful night, but we know anything can still happen," said Keane, who converted a header on a pass from Landon Donovan in first-half injury time before finishing his fourth goal of a dynamite post-season in the 67th minute. "So it's up to the lads, the senior players who have a lot of experience, to remind everybody that this is not over. We're happy with tonight, though."

The Galaxy entered the post-season as the West's fourth seed, but have maintained the impressive form they showed down the regular-season stretch. After beating Vancouver in a knockout playoff and ousting Supporters' Shield-winning San Jose in the first round, Los Angeles largely dominated the talented Sounders — particularly while scoring two goals in three minutes early in the second half.

If Los Angeles survives and Houston wraps up the Eastern Conference finals after beating D.C. United 3-1 earlier Sunday, the MLS Cup would be a rematch of last season's game, when David Beckham, Donovan and Keane won it all at Home Depot Center.

"I think when we're in really good form, we're hard to stop," said Magee, who has scored in six of the Galaxy's last eight playoff games dating to last season. "Having said that, we're already been stopped two times by this team in Seattle."

Indeed, Sounders coach Sigi Schmid immediately reminded his players that they beat the Galaxy 4-0 in their raucous home stadium Aug. 5.

"We have to do that again, but it's going to be a very difficult challenge," Schmid said. "We went through a bad period in the second half where we lost our shape, lost our concentration, lost our focus, and they banged in two goals. Robbie Keane and Landon Donovan were more dynamic up front than we were all day."

The Sounders played without defender Marc Burch, who is suspended three games for using a homophobic slur during Seattle's first-round series against Real Salt Lake.

Michael Gspurning made three saves to avoid an even more disastrous result for the Sounders, who must beat the Galaxy by four goals next weekend — or with a three-goal victory and penalty kicks after a 30-minute overtime — to advance to the franchise's first MLS Cup final.

"We're not foolish enough to believe this is over," Los Angeles coach Bruce Arena said. "But it was one of those nights where every player was on form."

Keane has been brilliant in his first full MLS season despite spending much of the year on planes for international duty as Ireland's captain at Euro 2012 and elsewhere. He also scored two goals in the second leg of the first round at San Jose, leading the Galaxy back from a first-leg deficit.

Magee scored the prettiest goal in a half full of a brilliant passing by the Galaxy in the 64th, with Beckham and Keane finding Franklin to set up Magee's third goal in Los Angeles' four playoff games.

"It was great that we put them away, but this tie is far from over," Beckham said. "We've still got to play our brand of soccer next week, and Seattle is always a tough place to play."

The Galaxy had most of the scoring chances on a chilly night south of Hollywood, but Seattle's Austrian goalie was sharp early, catching Donovan's header on the fly midway through the first half.

Los Angeles finally connected in the only minute of injury time on a play begun by Franklin. The defender came down the wing and centred it for Christian Wilhelmsson and Donovan, who chased it nearly to the end line before popping it in front.

Keane backpedaled, yet still had the body control and strength to head it past the sliding Gspurning. Keane did his trademark cartwheel-somersault goal celebration in front of the L.A. Riot Squad and turned to catch Donovan, who leaped into his arms.

The Galaxy were far superior after halftime, and a dazzling sequence of passing ended with an emphatic goal by Magee. Three minutes later, Keane pounced on a loose ball in front of Gspurning's net after Wilhelmsson's did the hard work.

"It was on its way in, but he's a striker," Wilhelmsson said with a grin. "That's how he should be."